The influence of the Extreme Ultraviolet spectral energy distribution on the structure and composition of the upper atmosphere of exoplanets

Abstract: By varying the profiles of stellar extreme ultraviolet (EUV) spectral energy distribution (SED), we tested the influences of stellar EUV SEDs on the physical and chemical properties of the escaping atmosphere. We apply our model to study four exoplanets, HD\,189733b, HD\,209458b, GJ \,436b, and Kepler-11b. We found that the total mass loss rates of an exoplanet, which are determined mainly by the integrated fluxes, are moderately affected by the profiles of the EUV SED, but the composition and species distributions in the atmosphere can be dramatically modified by the different profiles of the EUV SED. For exoplanets with a high hydrodynamic escape parameter ($\lambda$), the amount of atomic hydrogen produced by photoionization at different altitudes can vary by one to two orders of magnitude with the variation of stellar EUV SEDs. The effect of photoionization of H is prominent when the EUV SED is dominated by the low-energy spectral region (400-900${\AA}$), which pushes the transition of H/H$^{+}$ to low altitudes. On the contrary, the transitions of H/H$^{+}$ move to higher altitudes when most photons concentrate in the high-energy spectral region (50-400${\AA}$). For exoplanets with a low $\lambda$, the lower temperatures of the atmosphere make many chemical reactions so important that photoionization alone can no longer determine the composition of the escaping atmosphere. For HD 189733b, it is possible to explain the time variability of \lya\ between 2010 and 2011 by a change in the EUV SED of the host K star, yet invoking only thermal H\,I in the atmosphere.